The invention relates to an electroluminescent device having an electroluminescent element, which includes an electroluminescent organic layer, which contacts two electrodes, the electroluminescent element being enclosed in a housing.
An electroluminescent (EL) device is a device which, while making use of the phenomenon of electroluminescence, emits light when the device is suitably connected to a power supply. If the light emission originates in an organic material, the device is referred to as an organic electroluminescent device. An organic EL device can be used, inter alia, as a thin light source having a large luminous surface area, such as a backlight for a liquid crystal display or a watch. An organic EL device can also be used as a display if the EL device comprises a number of EL elements, which may or may not be independently addressable.
The use of organic layers as an EL layer in an EL element is known. Known organic layers generally include a conjugated, luminescent compound. This compound may be a low-molecular dye, such as a coumarin, or a high-molecular compound, such as a poly(phenylenevinylene). The EL element also includes two electrodes, which are in contact with the organic layer. By applying a suitable voltage, the negative electrode, i.e. the cathode, will inject electrons and the positive electrode, i.e. the anode, will inject holes. If the EL element is in the form of a stack of layers, at least one of the electrodes should be transparent to the light to be emitted. A known transparent electrode material for the anode is, for example, indium tin oxide (ITO). Known cathode materials are, inter alia, Al, Yb, Mg:Ag, Li:Al or Ca. Known anode materials are, in addition to ITO, for example, gold and platinum. If necessary, the EL element may comprise additional organic layers, for example, of an oxydiazole or a tertiary amine, which serve to improve the charge transport or the charge injection.
An EL device of the type mentioned in the opening paragraph is disclosed in a publication by Burrows et. al., published in Appl. Phys. Lett. 65 (23), 1994, 2922. The known device consists of an organic electroluminescent element which is built up of a stack of an ITO layer, an EL layer of 8-hydroxyquinoline aluminium (Alq.sub.3), a hole-transporting layer of N,N-diphenyl-N,N-bis(3-methylphenyl)1,1-biphenyl-4,4'diamine and an Mg:Ag layer, which is provided with a silver layer. Said EL element is surrounded by a housing consisting of a bottom plate and a top plate which are made of glass, said plates being interconnected by an epoxy-based adhesive for sealing. The ITO layer also forms the electrical leadthrough for the anode; the Mg:Ag/Ag layers also form the electrical leadthrough for the cathode. The leadthroughs are electrically insulated from each other by a layer of silicon nitride. The known device has disadvantages which render it unsuitable for use in durable consumer goods, such as a display or a backlight for a liquid crystal display or a watch. After several hours, a deterioration of the uniformity of the luminous surface occurs which can be observed with the unaided eye. The deterioration, which also takes place when the EL device is not in operation, manifests itself, for example, by so-called "dark spots" which are formed so as to be dispersed over the entire luminous surface. Also the presence of the housing itself gives rise to degradation of the EL device. For example, the epoxy-based adhesive gives off substances which are detrimental to the EL element. In addition, the manufacture of the EL device is time consuming. For example, the curing of a suitable epoxy-based adhesive takes 24 hours.